UTBOX and ground plane combined with Al2O3 inserted in TiN gate for VT modulation in fully-depleted SOI CMOS transistors

Thin film devices (FDSOI) are among the most promising candidates for next device generations due to their better immunity to short channel effects (SCE). In addition, the introduction of high-k and metal gate has greatly improved the MOSFETs performance by reducing the electrical oxide thickness (CET) and gate leakage current. However, if midgap metal gate is sufficient to provide a high symmetrical threshold voltage (V_T~0.45V) for both NMOS and PMOS devices [1], still one major challenge is to provide V_T modulation with an undoped channel in order to satisfy the low power (LP) circuit design requirements [2-5]. To overcome this issue, combining UTBOX substrate with ground plane (GP) has been proposed [2,5]. However this technique with midgap metal gate requires a FBB biasing in order to realize low V_T that´s implies a disruptive circuits design to avoid forward diode biasing in the substrate between the two opposite GP type beneath the BOX [6]. In order to introduce more V_T modulation flexibilities and especially for LVT PMOS and HVT NMOS, aluminum Oxide (Al₂O₃) inserted in TiN gate stack has been proposed for bulk devices [7-8] in a gate first process. The viability of this option is studied in this paper for FDSOI, for HfO₂ and HfSiON gate oxide, through transistors performance, reliability and variability analysis.